The signal recognition particle (SRP) is an essential ribonuclear protein complex responsible for the co-translational delivery of membrane and secretory proteins to the plasma membrane in bacteria and to the endoplasmic reticulum in eukaryotes. In Eubacteria, SRP consists of Ffh and a 4.5S RNA. In eukaryotes, the system is more complex, and is comprised of six proteins along with a large RNA moiety, the 7S RNA. Similarly, the eukaryotic SRP receptor (SR) consists of two proteins, SRα and SRβ, while in bacteria, a single protein, FtsY, is responsible for membrane association and interaction with the translocon. Specifically, the NG domain of FtsY (comprising the N-terminal and GTP binding domain) is involved in binding a GTP molecule and forming a heterodimer with the NG domain of Ffh. Structures of the bacterial SRP:SR complex revealed that the interactions with SR are mediated via extensive contacts at two sites on the 4.5S RNA, while in eukaryotes additional SRP protein components appear to fulfil the role of the 4.5S RNA.
Truncations or mutations to any of the bacterial components of the SRP system have proven to be either lethal or severely impact cell viability. This alongside the distinctions of the SRP:SR interaction between bacteria and eukaryotes make it an attractive drug target. As such, we propose specific targetting of the FtsY NG domain for novel antibiotic development. Using a Fragment-Based Drug Design (FBDD) approach coupled with Nuclear Magnetic Resonance (NMR) spectroscopy, we identified fragments that bind to FtsY. We have crystallized FtsY in an apo form, as well as in the presence of both GTP analogues and three fragments; with structures determined at resolution ranging from 1.3 – 1.8 Å. Development of SAR for the top fragment hits is ongoing using a combination of NMR, SPR and X-ray crystallography.